Condensation products of urea or its derivatives with formaldehyde and CH-acidic aldehydes have been disclosed in the patent literature. For example, according to German Pat. No. 1,272,284, the contents of which correspond to British Pat. No. 1,191,710, the reaction of the components in the stoichiometric ratio gives the ureido-propionaldehydes of the general formula ##STR3##
However, the products obtained cannot be used for certain applications, for example for the surface-coating industry. On storage, and especially on exposure to light, particularly on exposure to UV radiation, the products eliminate carbon monoxide and this causes the resins to froth.
It is an object of the present invention to provide a process which gives condensation products of a urea, formaldehyde and a CH-acidic aldehyde, which do not suffer from the above disadvantages.
We have found, surprisingly, that a further reaction of the condensation products of a urea, formaldehyde and a CH-acidic aldehyde with an alcoholate, preferably in the presence of an aromatic hydrocarbon, results in conversion to a resinous product which does not eliminate carbon monoxide.
Accordingly, the present invention relates to a process for the preparation of soft and hard resins from a urea formaldehyde and a CH-acidic aldehyde, by reacting a urea of the general formula (I) or (II) ##STR4## where R.sup.1 and R.sup.2 are hydrogen or identical or different alkyl, A is alkylene and X is oxygen or sulfur, with formaldehyde and a CH-acidic aldehyde of the general formula ##STR5## where R.sup.3 and R.sup.4 are identical or different alkyl, aryl or alkylaryl, at from 60.degree. to 150.degree. C., in the presence of an acid and in the presence or absence of a solvent and/or diluent, wherein the product is then treated with a base in an anhydrous medium.
In a preferred embodiment of the process according to the invention, the base used for the after-treatment is an alkali metal alcoholate in an anhydrous medium and the reaction is carried out in the presence of an aromatic hydrocarbon.
The present invention further relates to the use of the soft and hard resins prepared according to the invention, in combination with other film-forming agents, as surface-coating binders.
The soft and hard resins prepared by the process according to the invention do not eliminate carbon monoxide on storage and on exposure to light.
Depending on the selected molar ratios and on the aldehyde used, resins having softening points of from about 60.degree. to about 130.degree. C. are obtained.
No aldehyde groups, and also no carboxylic acid ester groups, alcohol groups or carboxylic acid groups, can be detected in the condensation products which have been after-treated with alcoholates. The products obtained have the character of a hard resin and are soluble in alcohol and in some cases even in white spirit.
The following details may be noted in respect of the components from which the resins obtained according to the invention are synthesized.
Suitable ureas are those of the general formulae (I) and (II) ##STR6## where R.sup.1 and R.sup.2 are hydrogen or identical or different alkyl, especially of 1 to 18, preferably of 1 to 5, carbon atoms, A is alkylene of 1 to 6, preferably of 1 to 3, carbon atoms and X is oxygen or sulfur.
In addition to urea or thiourea, monosubstituted and disubstituted ureas, and alkylenediureas, can also be employed. Preferably, however, urea is used to prepare the resins according to the invention.
The formaldehyde can be employed as an aqueous solution, for example of 40% strength, as paraformaldehyde, or as trixoane, tetroxane or a formaldehyde-acetal.
Since the condensation takes place via hydroxymethylureas, the urea and formaldehyde can be replaced by the corresponding hydroxymethylurea.
Suitable CH-acidic aldehydes of the general formula (III) ##STR7## where R.sup.3 and R.sup.4 are identical or different alkyl, aryl or alkylaryl are those with alkyl of 1 to 8, preferably of 1 to 6, carbon atoms, and those with aryl of 6 to 10 carbon atoms or alkylaryl of 7 to 10 carbon atoms.
For the purposes of the invention, CH-acidic aldehydes are aldehydes which carry a hydrogen on the carbon adjacent to the carbonyl group.
Amongst these CH-acidic aldehydes, isobutyraldehyde, a cheap compound, and 2-ethylhexanal deserve special mention. Further examples of CH-acidic aldehydes are 2-methylpentanal, 2-phenylpropanal and isovaleraldehyde.
In the process according to the invention, the urea, formaldehyde and CH-acidic aldehyde are in general employed in a molar ratio of from 1:2:2 to 1:4:4, the latter being preferred. The properties of the resins according to the invention, especially their softening points and compatibility with solvents, can be deliberately varied by varying the molar ratios. For example a molar ratio of 1:2:2 gives hard resins whilst a molar ratio of 1:4:4 gives products having the character of a soft resin.
Suitable acids are inorganic and organic acids, preferably sulfuric acid and oxalic acid.
Solvents and/or diluents may be used when carrying out the process according to the invention. Aromatic hydrocarbons, especially xylene, have proved to be particularly suitable solvents.
The acid condensation is carried out at from 60.degree. to 150.degree. C., preferably from 80.degree. to 100.degree. C., advantageously until complete conversion of the components is reached.
After completion of the acid condensation, the mixture is neutralized in the conventional manner with a base, eg. NaOH or KOH, and the water present is removed by distillation. Catalytic amounts, in general from about 2 to 5% by weight, based on solid resin, of a base, preferably an alkali metal alcoholate, are then added. Sodium methylate and sodium ethylate have proved particularly suitable for this purpose. The treatment with the base is preferably carried out at from 80.degree. to 120.degree. C. and requires from about 1 to 2 hours. The mixture is then neutralized with an inorganic or organic acid, eg. with sulfuric acid or phthalic acid, the salt is washed out with water and the resin, dissolved in the organic solvent phase, is freed from the solvent by distillation.
The preparation of the resins according to the invention may be carried out batchwise or continuously.
The soft and hard resins prepared according to the process of the invention may be used, in combination with other physically drying film-forming agents conventionally used as surface-coating binders, such as cellulose derivatives, eg. nitrocellulose, ethylcellulose, cellulose acetobutyrate, chlorinated rubber, copolymers based on vinyl esters, vinyl ethers, vinyl chloride, acrylic esters or vinyl-aromatics, eg. vinyl chloride/vinyl isobutyl ether copolymers, or chemically drying binders, eg. alkyd resins, drying oils and semi-drying oils, for the production of surface coatings. The content of resin, prepared according to the invention, in these surface coatings can be varied within wide limits but is in most cases not more than 50% by weight, preferably from 2 to 40% by weight, of the total binder.
The resins according to the invention can be processed using the solvent and diluents conventionally employed in surface coating, such as alcohols, ketones, ethers, esters, eg. butyl acetate, aliphatic and aromatic hydrocarbons, eg. toluene, xylene, hexane and white spirit, or mixtures of the above solvents, as well as pigments, dyes, plasticizers, flow control agents and other assistants.
The surface-coating binders of the invention may be employed for finishing substrates such as wood, chipboard, paper and metal, eg. sheet steel, using conventional methods of application, eg. brushing, spraying or casting.
The novel products have very advantageous processing characteristics. They give surface coatings having very good mechanical properties, good gloss, good light resistance and good water resistance. The surface-coating binders according to the invention may, for example, be used very advantageously in matt spray coatings, quick-drying primers, paper varnishes and anti-corrosion finishes.
In the Examples, parts and percentages are by weight.